The present invention relates to optical shaft position encoders which are used to generate electrical signals representative of the rotational position of a shaft. Encoders are used typically in control and monitoring systems as position sensors. Optical encoders utilize pairs of light sources amd semsors, such as photocells or other light sensitive devices, separated by transparent commutators arranged for rotation with the shaft and carrying coded opaque patterns for selectively interrupting the light paths according to shaft position.
Optical encoders were developed to avoid the problems resulting from the mechanical wear experienced by brush encoders in which an insulating commutator carries a coded pattern of conductors for selective contact with fixed brushes according to shaft position. The conventional optical encoder is provided with a fixed air gap between the sensors and the commutator pattern thereby avoiding any functional or wear problems.
Optical encoders are capable of great accuracy in sensing position. As the physical size of the commutator increses, the coded pattern may become more complex providing more detailed positional information. One problem associated with larger devices is the difficulty in maintaining the fixed air gap. This is critical to the accuracy of the encoders. The light sources used are not coherent sources so that variations in the air gap dimensions affect the integrity of the output signal.
In the newly emerging field of solar energy processing, an application for encoders has developed in which the maintenance of a constant air gap is a particular problem. In positioning large mirrors for directing reflected sun light against a target, such as a heat exchanger, high precision encoders are required. The mirrors are physically large, on the order of 400 square feet or more and are not protected from the winds. The torque effects of a strong wind on such a surface make it difficult to accurately maintain a small air gap, on the order of 0.005 inch, between a commutator mounted to the mirror and an optical head fastened to a mechanical base. In a conventional encoder these tolerance problems require the utilization of expensive optically flat glass.